What I would like to do is mash for a 10 gallon recipe. Then run off into separate kettles to make two 5 gallon batches using different hops and yeast. My thought is I would have to sparge into my keggle, stir, then split into the separate kettles. Is this practical? Does it make sense? Is there a better way?

I'm not looking for a partigyle with 1 big and 1 small beer. I'd like it to be split 50/50. The reasons I would like to do this is my 48qt rectangular mash tun gets better efficiency with 20lbs of grain as opposed to 10lbs. Also due to unexpected rain I found my stove can boil 6.5 gallons, so I'd like to try and have to separate kettles on the stove making two different beers from the same mash.

That's not partigyle, that's splitting a 10g batch. It'll work fine if that is what you are asking. It's also a great time to experiment with different hop schedules or yeasts.

This exactly what I want to do. I want to use the same grain bill, but use different hops and yeast.

I was thinking I would need to sparge into one vessel first then split. Easy enough.

The efficiency issue I believe has to do with temp loss due to all the extra head space. I assume. My mash tun is just overkill for 10lbs of grain. I don't believe I'm sparging more since the amount of sparge water depends on the amount of grain. Right?

I don't believe I'm sparging more since the amount of sparge water depends on the amount of grain. Right?

Yes, but if you break that amount of sparge water into several smaller sparges, you'll get better efficiency. By putting more grain into your mash tun, there is less space left for the sparge water; I assumed you might be doing multiple sparges because of this lack of volume.